During the period in which slavery was legal in the United States, the basic structure of the slave family differed somewhat from that of a free family. The overarching difference had to do with the fact that slaves were defined as property rather than people, and thus could not enter into legal contracts. Since marriage was a legal contract, slaves could not legally marry, although many entered into relationships quite similar to marriage and formed nuclear families.
Whether slave families could live together and enjoy a form of family life depended entirely on the whims of the owners. Some owners encouraged informal slave marriages (including a ceremony of "jumping the broom"); these relationships could be ones where slaves chose their partners or had their partners chosen for them by their owners. At any point, the owners were free to sell slaves, including parents and children, and thus separate families.
In some households, slave children were raised collectively by one or more slave women, outside of a nuclear family arrangement. Sometimes an older female slave would be responsible for child care while younger slaves did heavier physical labor.
Family separation was often cited as a major evil of slavery by abolitionists.